Serious performance issues

hi,

We’re running a science blog on WordPress, and we’re experiencing major performance issues with pages taking up to 25 seconds to load. The backend was also suffering up to the point our editors refused to work with it.

After evaluation, we decided to do the following:
– fresh WP-install with reimport of all content (no copy of DB)
– compressed all images
– switched host
– enabled server and client-side caching, GZIP compression, memcache, …
– enabled CDN
– minify
– disabled all plugins

The result: our backend is now acceptabel and most of the pages on our front-end as well. Except some pages that again take up to 25 seconds (including our homepage). Page testing tools reveal that the time to first byte can be up to 15 seconds on affected pages.

The question: how do we troubleshoot this?
– plugin vendors suggest conflicts and point to each other: we have tried disabling all plugins without any improvements
– the host suggests to enable a caching plugin: this does off course help but it only masks the problem. If we work on the page or the cache is not up to date, we cannot have people wait 25 seconds for the page
– after elimination, the only possible culprit is the theme.

Or are we missing something crucial?
Thanks!

https://www.heldenvandewetenschap.be